A conventional gas turbine engine typically includes a low pressure, or power, turbine having alternating rows of stators and rotors. The stators, also referred to as stator nozzles, include a plurality of circumferentially spaced apart stator vanes which suitably direct combustion gases to the rotor stage immediately downstream therefrom. Each rotor stage includes a plurality of circumferentially spaced apart rotor blades which extract energy from the combustion gases for rotating the rotor and in turn providing output shaft power therefrom.
A single stator stage typically includes a plurality of circumferentially adjoining stator segments each with a plurality of circumferentially spaced apart vanes fixedly joined to radially outer and inner bands. When assembled together, the outer and inner bands collectively form rings with a full complement of vanes extending therebetween. The stator segments must be securely mounted to the turbine outer casing for reacting axial, tangential, and bending forces imposed against the stator vanes by the combustion gases being channeled therebetween. These gases impose an axially rearwardly directed force against the vanes which in turn creates bending force, or moment, about the outer band which supports the stator segment to the casing. Since the vanes are pitched for suitably turning the combustion gases in the downstream axial direction, tangential forces are also imposed on the vanes which tend to rotate the stator assembly about the longitudinal centerline axis of the turbine.
Conventional mounting arrangements for securing the stator stages to the turbine casing include complementary forward hooks on the casing and the stator outer band which join together for radially retaining the stator segments to the casing at their forward ends. And, at their aft ends, the outer bands typically include an annular flange which abuts either a complementary casing flange or retention strip for reacting aft directed axial forces into the turbine casing. Anti-rotation pins or dowels are typically used to prevent the stator segments from rotating about the turbine centerline axis relative to the turbine casing by reacting the tangentially directed gas loads. Since the stator inner band is itself not supported directly to an adjacent structure, the stator vanes and the inner band tend to bend or tilt in the aft direction relative to the casing since they are cantilevered relative to the outer band mounted to the casing. Accordingly, the stator forward hook is also configured for reacting the bending forces acting on the stator.
During assembly of the stator segments to the turbine casing, each of the stator segments is typically tilted forward for assembling the forward hook to the turbine casing and for clearing the rear flange of the outer band with its complementary mounting flange in the casing. Tilting, however, requires additional annulus space in the turbine and, therefore, the turbine requires increased axial clearance between stators and rotors.
Accordingly, efficiency losses are introduced due to the increased annulus required for allowing tilting during assembly. And, the assembly process itself is relatively complex by requiring tilting and additional parts such as the tangential retention dowels and the axial retention strips. Furthermore, the dowels are typically provided through holes in the casing which effect undesirable stress concentrations which must be accommodated for maintaining useful life of the turbine casing. And, increased cost is also associated with these stator segment retention assemblies.